The present invention relates to a photographing device and a photographing method. More particularly, the present invention relates to a photographing device and a photographing method, in which the dynamic range magnification can be changed as desired and which can reliably photograph dynamic scenes, too.
Photographing devices incorporating a CCD (Charged Coupled Device) image sensor are widely used.
A CCD image sensor is a solid-state photographing element. It finds use in various photographing devices such as video cameras and digital still cameras. Photographing devices, each including a CCD image sensor, are widely used in parts inspection apparatuses in FA (Factory Automation) and optical apparatuses in ME (Medical Electronics), such as electronic endoscopes.
FIG. 1 shows the electrical configuration of an interline-type CCD image sensor for use in conventional photographing devices.
The image sensor includes a first-phase power supply 1, a second-phase power supply 2, and a third-phase power supply 3. The power supplies supply transfer pulses (drive voltages) to vertical-transfer CCDs (vertical-transfer registers) 7a and 7b via first-phase electrodes 4a to 4d (hereinafter referred to as “first-phase electrodes 4,” unless they need to be individually referred to), second-phase electrodes 5a to 5d, and third-phase electrodes 6a to 6d, respectively. The second-phase power supply 2 supplies a read pulse (drive voltage) to the vertical-transfer CCDs 7a and 7b, to read the charges accumulated in PDs (Photo Diodes) 8a to 8h. 
PDs 8 convert light constituting an image into electric charges and accumulate the electric charges. The charges accumulated in the PDs 8 are read into the vertical-transfer CCDs 7a and 7b when the PDs 8 receive the read pulses from the second-phase power supply 2. Only eight PDs 8 are shown in FIG. 1, arranged in four rows and two columns, for the sake of explanation. In fact, however, PDs 8 are arranged in far more rows and columns.
The vertical-transfer CCDs 7a and 7b have three polysilicon electrodes for each PD8. These electrodes function as a register that accumulates the electric charge read from the PD8. The polysilicon electrodes are arranged on the vertical transfer CCD 9, forming register cells that are arranged in the vertical direction and connected to one another. The charges accumulated in the registers provided beneath the polysilicon electrodes are transferred, each from an upper register to the immediately lower register. The charges are thereby output to a horizontal-transfer CCD 9 that is connected to one end of each vertical transfer CCD 7. The vertical-transfer CCDs 7 reads the charges output from the PDs 8 and output them to the horizontal-transfer CCD 9, such that the charges may not mix with one another.
First-phase electrodes 4, second-phase electrodes 5, and third-phase electrodes 6 are provided, each extending in the horizontal direction as illustrated in FIG. 1. When the second-phase electrode 5a, for example, supplies a read pulse, the charges in the PDs 8a and 8e in the same row are read to the vertical-transfer CCDs 7a and 7b, respectively, at the same time. Thus, the charges in the PDs 8 are read to the vertical-transfer CCDs 7 at the same time, because the PDs 8 are connected one electrode that extends in the horizontal direction. The transfer pulses are supplied to the horizontal-transfer CCD 9 at the same time the charges are supplied to the vertical-transfer CCDs 7, for the first-phase electrodes 4a to 4d, the second-phase electrodes 5a to 5d and the third-phase electrodes 6a to 6d. These transfer pulses make the vertical-transfer CCDs 7a and 7b transfer the charges from the PDs 8 to the horizontal transfer CCD 9.
The horizontal transfer CCD 9 is driven by the transfer pulses supplied from drive power supplies 10a and 10b. The CCD 9 receives the charges read from the PDs 8 and transferred from the vertical-transfer CCDs 7a and 7b and outputs these electric charges to an output terminal 11.
Various methods have been proposed, each using a CCD image sensor of the type described above, photographing images by using PDs 8 of different sensitivities and combining the images, thereby to enhance the dynamic range.
The first of these methods is disclosed in Jpn. Pat. Appln. Laid-Open Publication 8-223491, Jpn. Pat. Appln. Laid-Open Publication 7-254965, Jpn. Pat. Appln. Laid-Open Publication 7-254966, Jpn. Pat. Appln. Laid-Open Publication 8-340486, Jpn. Pat. Appln. Laid-Open Publication 10-069011, and U.S. Pat. No. 5,801,773. This method uses CCD image sensors arranged on the optical axes of incident light beams that pass through optical paths of different transmittances.
The second of the methods is disclosed in Jpn. Pat. Appln. Laid-Open Publication 8-331461, Jpn. Pat. Laid-Open Publication 7-254965, U.S. Pat. No. 5,420,635, U.S. Pat. No. 5,455,621, Jpn. Pat. Appln. Laid-Open Publication 6-141229, U.S. Pat. No. 5,801,773, U.S. Pat. No. 5,638,118, and U.S. Pat. No. 5,309,243. The method uses one CCD image sensor, which forms images of the same object at different times, each time for a different exposure time, and combines these images.
The third of these methods is disclosed in U.S. Pat. No. 5,789,737, Jpn. Pat. Appln. Laid-Open Publication 59-217358, and U.S. Pat. No. 5,420,635. This method uses one CCD image sensor that includes light-receiving elements having different sensitivities. The light-receiving elements convert light beams into electric signals, which are synthesized. To impart different sensitivities to the light-receiving elements, filters of different transmittances are bonded to the light-receiving elements.
The first method mentioned above requires a number of CCD image sensors and a complex optical system that splits one light beam into many light beams. The first method inevitably increases the size and manufacturing cost of the photographing device that adopts this method.
In the second method, one CCD image sensor generates data items about the same object at different sensitivities and different times, each time for a different exposure time. Therefore, the second method cannot photograph dynamic scenes (images) that change, in light intensity, from moment to moment.
The third method does not require a complex photographing device as the first method. Moreover, the third method can photograph dynamic scenes, unlike the second method. In the third method, however, the dynamic range magnification hardly can be adjusted in accordance with the use conditions, because each light-receiving element of the CCD image sensor has but a fixed sensitivity. It should be recalled that filters of different transmittances are bonded to the light-receiving elements, thereby imparting different sensitivities to the light-receiving elements.
Jpn. Pat. Appln. Laid-Open Publication 9-191099 discloses a method that may solve the problem that the fixed sensitivity of each light-receiving element makes it difficult to adjust the dynamic range magnification in accordance with the use conditions. In the method, signals are read from any selected column after the first accumulation period has elapsed, the CCD image sensor is then covered with an electronic shutter, and signals are read from another column during the second accumulation period. The method can indeed increase the dynamic range, but cannot photograph dynamic scenes which change, in light intensity, from moment to moment. This is because some time elapses between the first and second accumulation periods, since the electronic shutter discharges the photodiodes after the first accumulation period and before the second accumulation period. Consequently, the photographing time differs from column to column.
The method disclosed in Jpn. Pat. Appln. Laid-Open Publication 9-191099 has another problem. No freedom of designing the spatial pattern of sensitivity can be attained in the process of bonding the filters to the light-receiving elements. Consequently, the CCD image sensor cannot generate signals of three or more different types, each type generated for a different exposure time.
This problem is attributed to the following fact. The second-phase power supply 2 supplies read pulses via the second-phase electrodes 5. Charges are therefore read at the same time from the PDs 8 arranged in the horizontal direction (FIG. 1) into each vertical transfer CCD 7. The exposure time, i.e., the time for which each PD 8 of the CCD image sensor accumulates an electric charge until the charge is transferred from it, is the same for any horizontal line. The sensitivity of the PD 8 is determined by the exposure time. Hence, the PDs 8 arranged in the horizontal direction have the same sensitivity. This reduces the freedom of designing the spatial pattern of the PDs 8.
In the third method and the method disclosed in Jpn. Pat. Appln. Laid-Open Publication 9-191099, the light-receiving elements (PDs 8 shown in FIG. 1) of the CCD image sensor have information acquired at only one sensitivity, and the signals generated by the light-receiving elements are combined to provide an image of a broad dynamic range. The image thus provided has but a fraction of the resolution that the CCD image sensor can achieve.